Brooks said the gunman was carrying a semi-automatic rifle, and had opened fire from behind the dugout of the baseball field.

“He appeared to be a middle-aged white male,” Brooks added.

Witnesses say at least one congressman, Republican Whip Steve Scalise, was wounded by the gunfire. According to Fox News a congressional staffer for Scalise was also hit. CNN reports that police officers were also among the wounded.

The congressional baseball practice targeted by the gunman was a Republican event, Brooks said.

Steve Scalise - REUTERS

ABC 7 News reported that a suspected was “believed in custody”, while Senator Mike Lee told Fox News that the gunman had been shot and killed.

Bodyguards assigned to Scalise did open fire on the gunman, Brooks told CNN, but he did not state whether the gunman was hit.

Alexandria police confirmed via Twitter that the gunman was in custody.

Senator Jeff Flake (R-Arizona) said he witnessed the gunman being shot by police, but that his injuries had not been fatal.

White House Press Secretary Sean Spicer tweeted that both President Trump and Vice President Pence had been notified of the situation.

“Both @POTUS & @VP are aware of the developing situation in Virginia. Our thoughts and prayers are with all affected.”

President Trump released a statement in response to the incident, saying he and the Vice President were "deeply saddened" by the attack.

"The Vice President and I are aware of the shooting incident in Virginia and are monitoring developments closely. We are deeply saddened by this tragedy. Our thoughts and prayers are with the members of Congress, their staffs, Capitol Police, first responders, and all others affected."